vi . INTRODUCTION TO MECHANICS. 



. When the attraction of cohesion and that of gravitation are opposed to 

 each other, the former, within the limits at which it acts, is generally 

 the stronger. Of this we have an instance in the attraction of capillary 

 tubes, in which liquids ascend by the attraction of cohesion, in opposition 

 to that of gravity. It is, however, necessary that the bore of the tube 

 should be extremely small ; for if the column of water within the tube is 

 not very minute, the attraction would not be able either to raise or sup- 

 port its weight, in opposition to that of gravity. It may be observed, also, 

 that all solid bodies are enabled, by the force of the cohesive attraction of 

 their particles, to resist that of gravity, which would otherwise disunite 

 them, and bring them to a level with the ground, as it does in the case of 

 liquids, the cohesive attraction of which is not sufficient to enable them 

 to resist the power of gravity. There is no attraction of cohesion between 

 the separate parts of pulverized bodies ; every grain of powder or sand is 

 composed of a great number of more minute particles, firmly united by 

 the attraction of cohesion ; but amongst the grains themselves there is no 

 sensible attraction, because they are not in sufficiently close contact. 



The surface of bodies is so rough and uneven, that, when in actual 

 contact, they touch each other only by a few points. Thus, if a book, 

 the binding of which appears perfectly smooth, be laid on the table, so 

 few of the particles of its under surface come in contact with the table, 

 that no sensible degree of cohesive attraction takes place ; it does not 

 stick, or cohere to the table, and there is no difficulty in lifting it off. It is 

 only when surfaces perfectly flat and well polished are placed in contact, 

 that the particles approach in sufficient number, and closely enough to 

 produce a sensible degree of cohesive attraction. Take two hemispheres 

 of polished metal, press their flat surfaces together, having previously 

 interposed a few drops of oil, to fill up every little porous vacancy. It 

 now requires a weight of several pounds to separate them : but part of 

 this effect is due, as will be explained hereafter, to a pressure of the air 

 on their surface ; the residue is the effect of cohesion. The same cause 

 which occasions the fall of bodies produces also their weight; in other 

 words it is the attraction of gravity which makes bodies heavy. The power 

 which brings bodies that are unsupported to the ground causes those which 

 are supported to press upon the objects which prevent their fall, with a 

 weight equal to the force with which they gravitate towards the earth. 



Attraction being mutual between two bodies, when a stone falls to the 

 earth, the earth should rise part of the way to meet it. But when, on 

 the other hand, you consider that attraction is proportioned to the mass of 

 the attracting and attracted bodies, you will no longer expect to see the 

 earth rising to meet the stone. You may possibly imagine that, according 

 to this theory, the hills should attract the houses and churches towards 

 them. The hills no doubt exert this influence, but they cannot move the 

 buildings, because they can neither overcome the attraction of cohesion 

 between the bricks and the mortar, nor that of gravity which fixes the wall 

 to the ground. There are, however, some instances in which the attrac- 

 tion of a large body has sensibly counteracted that of the earth. If a man, 

 standing on the declivity of an abrupt mountain, hold a plumb-line in his 

 hand, the weight will not fall perpendicularly to the earth, but incline a little 

 towards the mountain ; and this is owing to the lateral or sideway attraction 

 of the mountain interfering with the perpendicular attraction of the earth. 



If no obstacle intervened to impede the fall of bodies, attraction would 

 make them all descend with an equal velocity, or quickness ; so that 

 those which fall from the same height would reach the earth in the same 



